Text fields in formspider are < input > elements. You must attach an event to each existing and future input element, by using the (deprecated now) live function.
I modified my main.jsp, and add a reference to a custom js file. inside this file added the following function:
function enableSelectAll() {
$("input").live("focus", function(){
this.select();
});
}
Add a post-open application event that calls the following action:
procedure setSelectText is
fParams formspider.api_externalmethod.tt_params;
begin
formspider.api_externalmethod.invoke('setSelectText', fParams);
end;
Don't forget to declare the external-event named setSelectedText.

You can further improve the function by using more specific jquery selectors. For example using the styleClass property and select only that class.